CAS Registry Numbers are an important tool for substance searching. These are unique identifiers assigned by the Chemical Abstracts Service (CAS) to specific substances identified by them in the chemical literature.

CAS Numbers are not related to chemistry, are unrelated to any previous systems, and do not readily form phonetic analogs or synonyms. The numbers are simple and regular, convenient for database searches.

The format of CAS number is three blocks of numbers divided by hyphens ie. XXX-XX-X. It should will always be read from right to left:

The first block can be between 2 and 6 digits. The second block has only 2 digits and the last block is always a single digit. Any preceding zeros in the first block need to be discarded. For example, the CAS number for formaldehyde is 50-00-0.

Cas numbers save you from having to think of all the different trivial or semisystematic names, synonyms, brand names, or possible systematic names when you are doing a compound search by linking them all into a single search term.

For example, there are over ten synonyms for methanol, e.g., carbinol, methyl hydroxide, methyl alcohol, wood alcohol. The CAS Registry Number for it, 67-56-1, is specific to all those names for the same substance.

Where to find CAS numbers on the Internet? The following free websites can be used:
1) Chemical Synthesis Database
2) Common Chemistry
3) R&D Chemicals
4) Pubchem
5) ChemIDplus